If you are one of those readers that love to spot an Easter Egg in a comic book then this is something that is truly astounding.
However they are now exceedingly rare, due to a massively long and vastly complicated copyright dispute (which makes it even more interesting to find it in DKR) and low print runs. This means that these very important comic books are probably un-publishable and, sadly, rarely read. Which is a shame, because these books are truly awesome and helped to usher in the Modern Age.
I'm sure that this is Millers tribute, and I can only assume he got away with it, as he was able to finish the book away from editorial control. As is explained in the DKR afterword, DC editorial granted Miller a lot of lee-way towards the of the DKR run simply because they trusted Miller to complete the story on deadline without seeing the pencils.
Nice one Frank!